Numerous processes have been proposed to remove hexavalent chromium from aqueous solutions. Examples of industrial solutions that require removal of hexavalent chromium include: chromium plating baths, etching solutions, electrochemical machining solutions, rinse waters, and waste waters.
Methods employed in industry to remove chromium from various solutions can be classified as ion exchange or adsorption (see U.S. Pat. Nos. 3,835,001; 4,376,706; and 4,952,320) ;electrochemical generation of ferrous ions; direct precipitation of chromium with a salt dissolved in the solution (see U.S. Pat. Nos.3,969,246; 4,481,090; and 5,098,579); and chemical reduction of hexavalent chromium to trivalent chromium, followed by precipitation of the trivalent chromium.
Ion exchange is a unit process by which ions of a given species are displaced from an insoluble exchange material by ions of a different species in solution. The chromium-containing solution enters one end of the column under pressure, passes through the resin bed, and the chromium is removed from the solution. When the resin capacity is exhausted, the column is backwashed to remove trapped solids and then regenerated.
A disadvantage of an ion exchange method for chromium removal is that ion exchange resins are very selective. A resin must be chosen that selectively removes the metal contaminant of concern. Further, ion exchange equipment can be expensive and there can be incomplete removal of the chromium from the salt solution.
Another method for removing chromium from solution is the electrochemical generation of ferrous ions. This involves passing a liquid stream containing heavy metals at a controlled rate through a small gap between cold-rolled steel electrodes. The direct current passing through these electrodes forms ferrous iron at the anode, while hydroxide and hydrogen gas are produced at the cathode. The ferrous iron reduces the hexavalent chromium to trivalent chromium and in turn is oxidized to ferric iron. For every three ferrous ions oxidized one hexavalent chromium is reduced. The trivalent chromium, ferric, and leftover ferrous ions then react with the hydroxide being formed at the cathode resulting in insoluble chromium and iron hydroxides.
This process suffers from the shortcoming of utilizing costly and complex equipment. Adjustment of the pH to a neutral range, and possibly an acidic range may be required. Also, another drawback is that the process may not work with nitrate solutions.
Another method for chromium removal from aqueous solutions is the direct precipitation of chromium as chromate, utilizing cations.
U.S. Patent No. 3,969,246 of Feltz and Cunningham discusses a process where chromium is removed from waste water by direct precipitation using barium carbonate in aqueous solutions acidified with glacial acetic acid, followed by filtration of the resultant chromium material.
Childs describes, in U.S. Pat. No. 4,481,090, chromium removal from an electrolytic solution that is used to decontaminate radioactive surfaces. Lead or barium cations are added to the solution. The chromium ions in solution are precipitated as lead chromate or barium chromate.
In other known processes for removal of chromium from aqueous solutions, chemical reduction has been used to reduce hexavalent chromium to trivalent chromium. It is known that ferrous sulfate is often used to reduce hexavalent chromium to trivalent chromium. Chromic hydroxide is formed with iron hydroxides and is precipitated under alkaline conditions. The resultant precipitate is removed by conventional means such as filtration.
Peterson and Dexter describe, in U.S. Pat. No. 3,616,344, the reduction of hexavalent chrome in an alkali metal chlorate solution used for electrochemical machining. The solution is treated with either a ferrous salt, an alkali metal, ammonium sulfide, or a stannous salt to reduce hexavalent chromium to the trivalent chromium. The chromium precipitates from solution as hydrous chromic oxide.
Prior art processes for the removal of hexavalent chromium from aqueous solutions are effective for the purpose intended, but they have disadvantages. The ion exchange and electrochemical generation of ferrous ion processes utilize costly and complex equipment. The direct precipitation method generally leaves the chromium in the toxic hexavalent state. Both the direct precipitation of chromium and the chemical reduction of hexavalent chromium to trivalent chromium, followed by precipitation of trivalent chromium, introduce unwanted ions into solution. The prior art does not provide a method to reduce hexavalent chromium to trivalent chromium, remove chromium from the aqueous electrolyte solution,and regenerate the solution without introduction of unwanted ions.
There is a need to develop a method that reduces hexavalent chromium to trivalent chromium in aqueous electrolyte solutions, removes the trivalent chromium from the solution, and restores the solution to its original condition free of unwanted ions.